Thick film materials are mixtures of metal, glass and/or ceramic powders dispersed in an organic medium. These materials are applied to nonconductive substrates to form conductive, resistive or insulating films. Thick film materials are used in a wide variety of electronic and light electrical components.
The properties of individual compositions depend on the specific constituents which comprise the compositions. All compositions contain three major components. The conductive phase determines the electrical properties and influences the mechanical properties of the final film. In conductor compositions, the conductive phase is generally a precious metal or mixture of precious metals. In resistor compositions, the conductive phase is generally a metallic oxide. In dielectric compositions, the functional phase is generally a glass or ceramic material.
The binder is usually a glass which holds the film together and binds it to the substrate. The binder also influences the mechanical properties of the final film.
The organic medium or vehicle is a solution of polymers in organic solvents. The vehicle determines the application characteristics of the composition.
In the composition, the functional phase and binder are generally in powder form and have been thoroughly dispersed in the vehicle.
Thick film materials are applied to a substrate. The substrate serves as a support for the final film and may also have an electrical function, such as a capacitor dielectric. Substrate materials are generally nonconducting.
The most common substrate materials are ceramics. High-purity (generally 96%) aluminum oxide is the most widely used. For special applications, various titanate ceramics, mica, beryllium oxide and other substrates are used. These are generally used because of specific electrical or mechanical properties required for the application.
In some applications where the substrate must be transparent--such as displays--glass is used.
Thick film technology is defined as much by its processes as by the materials or applications. The basic thick film process steps are screen printing, drying and firing. The thick film composition is generally applied to the substrate by screen printing. Dipping, banding, brushing or spraying are occasionally used with irregular-shaped substrates.
The screen printing process consists of forcing the thick film composition through a stencil screen onto the substrate with a squeegee. The open pattern in the stencil screen defines the pattern which will be printed onto the substrate.
After printing, the film is dried and fired--generally in air at a peak temperature of 500.degree.-1000.degree. C. This process forms a hard, adherent film with the desired electrical and mechanical properties.
Additional thick film compositions may be applied to the same substrate by repeating the screen printing, drying and firing processes. In this way, complex, interconnected conductive, resistive and insulating films can be generated.
Thick film resistor compositions are usually produced in decade resistance values and materials are available that provide a wide range of sheet resistance (0.5 .OMEGA./.quadrature. to 1.times.10.sup.9 .OMEGA./.quadrature.). A change in aspect ratio, length to width, of a resistor will provide resistance values lower than 0.5 .OMEGA./.quadrature. and higher than 1.times.10.sup.9 .OMEGA./.quadrature. and any intermediate resistance value.
Composition blending is a technique widely used to obtain a resistance value between standard decade values. Adjacent decade members can be mixed in all proportions to produce intermediate values of sheet resistance. The mixing procedure is simple but requires care and the proper equipment. Usually blending has minimal effect on Temperature Coefficient of Resistance (TCR).
High electrical stability and low process and refire sensitivity are critical requirements for thick film resistor compositions for microcircuit applications. In particular, it is necessary that the resistance (R) of the films be stable over a wide range of temperature conditions. Thus, TCR is a critical variable in any thick film resistor composition. Because thick film resistor compositions are comprised of a functional or conductive phase and a permanent binder phase, the properties of the conductive and binder phases and their interactions with each other and with the substrate affect both resistivity and TCR.
Since copper is an economical electrode material, there is a need for thick film resistor systems which are compatible with copper and fireable in a nonoxidizing atmosphere and which have properties comparable to air fired resistors. Among the resistance materials which have been suggested for this purpose are lanthanum hexaboride, yttrium hexaboride, rare earth hexaborides and alkaline earth hexaborides. In this regard, Baudry et al. in French Pat. No. 2,397,704 have suggested resistance materials which are stable in a nonoxidizing firing atmosphere comprising an admixture of finely divided particles of a metal hexaboride and a glass frit which is an alkaline earth metal boroaluminate. In the Baudry patent, it is disclosed that the glass, which does not react with metal hexaborides, may contain no more than about 1% by volume metal oxides which are reducible by the metal hexaboride. Furthermore, in applicant's EPO Patent No. 0008437 are disclosed resistance materials which are comprised of an admixture of finely divided particles of metal hexaboride and a glass which is not reducible by the metal hexaboride. In this patent, it is disclosed that the glass may contain no more than 2 mole % of reducible metal oxides. In addition, U.S. Pat. No. 4,225,468 to Donohue is directed to similar hexaboride resistance materials comprising an admixture of finely divided particles of metal hexaboride, nonreducing glass and various TCR modifiers dispersed therein in particulate form, including particles of TiO and NbO.
Izvestia Vysshikl Uchebnykl Zavendenii, Nefti y Gaz, 16 (6), 99-102 (1973), discloses thick film resistors based on relatively coarse LaB.sub.6 and borosilicate glass. These resistors are said to be resistant to hydrogen gas; however, the films are moisture sensitive.
British Pat. No. 1,282,023, published July 19, 1972, discloses electrical resistor dispersions containing rare earth of alkaline earth hexaboride conductive pigment and a glass phase dispersed in ethyl cellulose medium. The glasses used are lead borosilicates as well as lead aluminoborosilicates, the latter of which is shown to contain as little as 16 mole % of hexaboride reducible oxides of low melting metals such as Pb, Na, Co and Ni. While such metal hexaboride-based resistors have been found to be quite useful, they nevertheless have also been found to be somewhat limited in their power handling capability, especially when they are formulated to make resistance materials in the 1K-100K ohm range. More recently, Francis-Ortega in U.S. Pat. No. 4,420,338 discloses resistors of metal hexaborides containing alkaline earth silicoborate glasses modified with small amounts (less than 5 mole %) of reducible oxides of V, Nb and Ta. The purpose of the reducible oxide is purported to be to improve TCR. However, it has been found that such oxides react with the hexaborides to form either diboride particles or metals which progressively lower the resistance. This process instability is shown by excessive lowering of the resistance on refiring.